Technical Field
The present disclosure relates to a control apparatus for a motor that is driven by an alternating-current voltage outputted from an inverter.
Related Art
As this type of control apparatus, a control apparatus is known in which an output torque of a motor is controlled to a command torque by an inverter being operated based on overmodulation pulse width modulation (PWM) control or 180-degree rectangular wave control. In overmodulation PWM control, both amplitude and phase of an output voltage vector of the inverter can be controlled. Meanwhile, in 180-degree rectangular wave control, the output voltage of the inverter is fixed at a maximum value thereof. Therefore, only the phase of the output voltage vector is a manipulated variable.
Here, a significant difference is present between the maximum voltage that can be outputted from the inverter under overmodulation PWM control and the voltage outputted from the inverter under 180-degree rectangular wave control. As a result of this difference, there is a voltage range that cannot be outputted from the inverter. Thus, there is a current range that is unattainable when a current vector of the current flowing to the motor is in a steady state. When a command current vector corresponding to the command torque is included in the unattainable current range, a hunting phenomenon occurs. In the hunting phenomenon, control mode is frequently switched between overmodulation PWM control and 180-degree rectangular wave control. Consequently, torque fluctuations occur in the motor. Therefore, switchover conditions for switching the control mode from overmodulation PWM control to 180-degree rectangular wave control, and from 180-degree rectangular wave control to overmodulation PWM control, are required to be appropriately set.
Here, in JP-A-2015-12662, less-than-180-degree rectangular wave control is provided between overmodulation PWM control and 180-degree rectangular wave control. In addition, in JP-A-2015-12662, when the control mode is switched from 180-degree rectangular wave control to overmodulation PWM control, through less-than-180-degree rectangular wave control, a condition that a comparison is performed between the current vector and a threshold thereof, or a condition that a comparison is performed between a d-axis current flowing to the motor and a threshold thereof is provided. As a result, occurrence of the hunting phenomenon is suppressed.
Here, to reliably prevent the occurrence of the hunting phenomenon, setting the above-described threshold so as to have a large margin in relation to an operation line of the command current vector can also be considered. However, in this case, a problem arises in that current fluctuations in the motor that occur when the control mode is switched from rectangular wave control to overmodulation PWM control increase. In this way, there is still room for improvement in techniques for suppressing the occurrence of the hunting phenomenon.